The present invention relates to a method and an appropriately equipped apparatus for transferring data according to an ARQ method, especially a hybrid ARQ method in a communication system; in particular, a mobile radio system.
Especially in conjunction with a mobile radio system, packet-access methods or packet-oriented data connections are proposed as the incoming message types often have very high burst factors so that there are only brief periods of activity interrupted by long periods of inactivity. In this case, packet-oriented data connections can be considerably more efficient when compared with other data transfer methods where there is a continuous stream of data, since in data transfer methods with a continuous data stream a resource (e.g., a carrier frequency or a time slot), is assigned for the entire communication process (i.e., the resource remains tied up even if there is temporarily no data transfer), with the result that the resource is not available for other network users. This results in non-optimum use of the limited frequency spectrum for mobile radio systems.
Future mobile radio systems such as those according to the UMTS mobile radio standard (Universal Mobile Telecommunications System), will offer a number of services in which multi-media applications will become increasingly significant in addition to pure voice transmission. The associated number of services with different transfer rates requires a very flexible access protocol on the radio interface of future mobile radio systems. Packet-oriented data transfer methods have proved to be suitable here.
In conjunction with UMTS mobile radio systems, an ARQ method (Automatic Repeat Request) was proposed for packet-oriented data connections. In this method, the data packets sent from a sender to a receiver are checked for quality after decoding on the receiver side. If a received data packet is errored, the receiver requests a new transfer of this data packet from the sender; i.e., a retransmission data packet is sent from the sender to the receiver that is either identical or partially identical to the data packet sent and the errored data packet received. Depending on whether the retransmission packet contains the same or less data than the original data packet, the retransmission is described as either complete or partial. This ARQ method proposed for the UMTS mobile radio standard, also known as a hybrid ARQ type I method, is proposed not only for transferring data but also for transferring header information in a data packet whereby the header information can contain information coded for error checking such as CRC bits (Cyclic Redundancy Check) and for error correction (known as Forward Error Correction, FEC).
According to the current UMTS standard, the proposal is to transfer the bits of the individual data packets or retransmission data packets after the channel has been appropriately coded using QAM modulation (Quadrature Amplitude Modulation). The individual bits are mapped using a method known as “gray mapping” to appropriate QAM symbols that create a two-dimensional symbol field. The problem with the proposed QAM modulation with an alphabet set that encompasses more than four QAM symbols is that the reliability of the bits to be transferred between the high-value bits and the low-value bits varies considerably. This is particularly disadvantageous with regard to channel coding since the turbo coders preferably used demand a high bit reliability to achieve a sufficiently high performance. With the hybrid ARQ type I method described, in which the retransmission data packet is identical to the original data packet, the variation in the bit reliability already described means that certain bits of the data packets and the retransmission data packets are found in the same position in the QAM symbol field. This reduces the overall performance of the data transfer and leads to premature restriction on the data throughput rate.
To resolve this problem, a method is proposed whereby those bits which occur in the same position in the original data packet and in the retransmission packet are assigned different QAM symbols in the QAM symbol field through dynamic rearrangement of the gray mapping.
This is described in more detail below with reference to FIGS. 4A-4D. FIG. 4A shows the signaling constellation or QAM symbol field for a 16-QAM modulation. Bits i1 and i2, and q1 and q2 are mapped to an appropriate QAM symbol 26 in the two-dimensional QAM symbol field 25 in the order i1 q1 i2 q2. The possible columns or rows of QAM symbols 26 for each of the bits i1, i2, q1, q2 in the two-dimensional QAM symbol field 25 are each marked using appropriate lines. So, for example, bit i1=“1” can only be mapped to the first two columns of the QAM symbol field. Due to the gray mapping, the reliability of the higher value bit i1 is greater than the lower value bit i2. Furthermore, the bit reliability of the bit i2 varies depending on the QAM symbol 26 that is transferred; i.e., depending on whether the relevant QAM symbol 26 is assigned to the outside left or outside right column of the QAM symbol field 25. The same applies to bits q1 and q2, as the mapping of bits q1 and q2 is equivalent to the mapping of bits i1 and i2 (albeit orthogonal).
According to the conventional method described with reference to FIGS. 4A-4D, it is proposed to use a gray mapping for retransmission data packets that is distinct from the gray mapping for the original data packet; i.e., gray mapping (for example, as depicted in FIG. 4B) can be used for an initial retransmission data packet, while for a second retransmission data packet, gray mapping as shown in FIG. 4C, and for a third retransmission data packet, gray mapping as shown in FIG. 4D, can be used. When comparing the representations in FIGS. 4A-4D, it is clear that the same bit combination i1 q1 i2 q2 is always assigned different QAM symbols 26; i.e., different points in the two-dimensional QAM symbol field 25. This dynamic variation of the gray mapping can be extended such that a specified number of retransmissions of each bit i1, i2, q1 and q2 can be transferred with very good, good or poor reliability to a point in the QAM symbol field 25 whereby this method can be optimized for a different number of retransmissions.
It is clear from FIGS. 4A-4D that this method is relatively complicated as the gray mapping has to be changed for each retransmission data packet.
An object of the present invention, therefore, is to propose a method and an appropriately equipped apparatus to transfer data according to an ARQ method to resolve the problem described above; i.e., to achieve a reliable data transfer with a high data throughput rate in the simplest manner.